Ion sensors are designed to detect chemical substances with the use of a membrane having a cyclic compound called the ionophore or ion exchange resin fixed thereon. Such sensors detect chemical substances with excellent specificity. As one example, there is available an ion sensor for the measurement of the concentration of ions in a sample under examination, which uses an ion sensitive field-effect type transistor (hereinafter often abbreviated as ISFET).
The ISFET makes use of a conductivity change in the vicinity of a semiconductor surface, which occurs depending upon a field change on the interface of a solution and a sensitive membrane provided on the surface of that semiconductor.
Such ISFET's use of ion sensitive membrane formed on an insulating film on the gate of a field-effect type transistor (hereinafter often abbreviated as FET). There is also known an extended gate type ISFET of a structure where the ion sensitive membrane is isolated from the FET on the same semiconductor substrate.
As the ion sensitive membranes for these ISFET's , there are known hydrogen ion sensitive membranes formed of an inorganic material such as silicon nitride, aluminum oxide or indium oxide. Alternatively, use is made of an ion sensitive membrane formed of an organic material such as, for instance, a polyvinyl chloride membrane carrying thereon an active substance--called an ionophore--which selectively entrains ions. For instance, potassium and sodium ion sensitive membranes may be formed by using as the ionophores valinomycin and bis-(12-crown-4), respectively.
Just after preparation, an electrode coated with an ion sensitive membrane needs to be immersed and stored in an aqueous ion solution of the type to be examined. This is necessary to insure the stable operation of the ion sensitive membrane. Unfortunately, however, a problem may develop in that the polyvinyl chloride membrane does not adhere well to the surface of the electrode. This often causes the ion sensitive membrane to peel. Alternatively, vesicles or blisters may form along the interface between the membrane and the surface on which it is coated. Either of these problems serves to degrade the overall characteristics of the ion sensor. This is especially true of an ion sensor of the extended gate type ISFET structure where the gate electrode of the FET is extended to isolate the ion sensitive membrane from the FET portion through an extended connecting line. Where peeling or blisters occur, the floating capacity of the gate electrode is increased, resulting in decreases in its response rate.
In addition to the problems with the aforesaid ion sensitive membranes, there has been another problem with gate electrodes.
Namely, a metal such as silver or gold or an oxide such as IrO.sub.2 or SnO.sub.2 have heretobefore been used for an extended gate formed by extending the gate of the FET. With such an extended gate ISFET, it is virtually impossible to obtain a certain or higher current, e.g., on the order of micro-amperes due to the fact that a potential occurring on the ion sensitive membrane depends upon polarization. For that reason, there is a need of using a transducer having a high input impedance, while a floating capacity from the ion sensitive membrane to the gate electrode correlates with response rates. The latter point is of no substantial significance in the case of mounting the ion sensitive membrane on the gate insulating film but, in the case of the structure where the ion sensitive membrane is separated from the FET portion through a long connecting line, offers a problem that the floating capacity is too increased to decrease the response rates.